Both workbook protection and sheet protection can be removed from an Excel workbook file without the protection password in four basic steps:

1. Unzip the .xlsx or .xlsm file so that its contents may be modified in the following steps. Excel workbook files are actually ZIP files with a different file name extension.
2. Remove the <workbookProtection ... /> XML tag from the xl\workbook.xml file.
3. Remove the <sheetProtection ... /> XML tag from any .xml file in the xl\worksheets\ directory.
4. Zip the modified Excel workbook file contents, using the .xlsx or .xlsm filename extension for the resulting ZIP file.

Yes. Excel workbook files are governed by the ECMA-376 standard, which clearly states that workbook protection and sheet protection are not intended to provide file security. To read this in the original, see Section 18.2.29 of ECMA-376-1:2016, which can be found at the link titled "ECMA-376 5th edition Part 1" on the official ECMA-376 Standard webpage.

Removing Excel Protection is a PDF guide (with screenshots!) for removing both workbook protection and sheet protection from an Excel workbook file using only built in Windows utilities (File Explorer & Notepad).